Currently, in third generation partnership project (3GPP), standardization of a wireless communication system for 4G has been promoted. In 4G, a technology such as relay, carrier aggregation, coordinated multiple point transmission and reception (CoMP), and multi user multi input multi output (MU-MIMO) has been attracting attention.
Relay is considered as an important technology to improve a throughput of a cell edge. In addition, carrier aggregation is a technology that can handle a band width of 20 MHz×5=100 MHz by handling, for example, five frequency bands each having a band width of 20 MHz, together. By such carrier aggregation, improvement of a maximum throughput can be expected.
In addition, CoMP is a technology in which a plurality of base stations transmit and receive data in cooperation in order to improve coverage of a high data rate. In addition, MU-MIMO is a technology that improves a system throughput so that a plurality of users use a resource block of the same frequency and the same time, on which spatial multiplexing is performed. As described above, further improvement of the performance in 4G (LTE-Advanced) by various technologies has been discussed.
Here, MU-MIMO is described in detail. In 3.9G (LTE), there are technologies of MU-MIMO and single user MIMO (SU-MIMO). For example, as discussed in Patent literature 1, SU-MIMO is a technology in which a plurality of channels are used so that single user equipment (UE) performs spatial multiplexing of the plurality of channels although spatial multiplexing is not performed between pieces of UE.
On the other hand, as described above, MU-MIMO is a technology in which each UE uses a resource block of the same frequency and the same time, on which spatial multiplexing is performed (spatial multiplexing is performed between pieces of UE). However, in MU-MIMO that is realized in 3.9G, each UE handles a mere single channel. On the contrary, in 4G, MU-MIMO in which each UE can handle a plurality of channels is being realized.
In order to achieve such MU-MIMO in 4G, it has been studied that two types (V1 and V2) of transmission weight are used in a base station. The V1 is transmission weight that realizes directivity, and the V2 is transmission non-directional weight, the main purpose of which is to adjust a phase. The V1 and V2 can be determined, for example, in UE. To be more specific, the UE receives a reference signal that is transmitted from a base station, obtains a channel matrix H from the reception result of the reference signal, and determines optimal V1 and V2 for the channel matrix H.